1. Technical Field of the Invention
The present invention relates to .gamma.-lactone derivatives and a process of preparing the .gamma.-lactone derivatives. In particular, the present invention relates to .gamma.-lactone derivatives which are useful as a starting material for synthesis of brassinolide, epibrassinolide and bisnorbrassinolide and derivatives thereof, and also a process of preparing the same.
2. Description of the Prior Art
Recently, the study of the synthesis of brassinolide which has the plant growth promoting activity and brassinosteroids which are analogues of brassinolide has been actively carried out in the hope of producing the brassinolide and derivatives thereof with high stereoselectivity.
Many methods for synthesis of brassinosteroids have been reported to date, and in most of which utilize pregnane-20-carboxaldehyde as a starting material, and comprise a step of introducing carbon chain into such starting material. Accordingly, in adopting such conventional methods, the following defects were found: The by-production of stereoisomer is inevitable; On subsequent introduction of oxygen-functional group, reduction, introduction of alkyl group, special reactant and expensive reagents must be used.
As a typical example of such method, it was disclosed in J. Chem. Soc. Perkin I, 1983, 383. As disclosed in the literature, the method is carried out in the accordance with the reaction process described below; The carbon chain is introduced into pregnane-20-carboxaldehyde (a) by means of a special Grignard reregent (b). ##STR1## In this method, the product is obtained in the form of mixture of stereoisomers (c) and (d) (3:2) and accordingly the stereoselectivity is low and the resolution of the mixture is complicated.
In addition, the method disclosed in J. Ame. Chem. Soc. 102, 6580 (1980) is accomplished through the following reaction process. ##STR2##
According to this method, unnecessary isomer (i) is by-produced in the reduction of compound (g) with LiBH.sub.4.BF.sub.3.